A friction-welding machine generally comprises two clamping devices in which respective workpieces are fixed and so juxtaposed that the workpieces lie in contact with one another along an interface at which a friction weld is to be formed.
One of the clamping devices and hence one of the workpieces is rotated rapidly against the other workpiece, which can be stationary, and the pressure of the cylinder arrangement is applied transversely to the interface.
Friction welding is an economical and convenient way of welding two workpieces together with a high-quality precision bond and is used when at least one side of the weld cross section is to be rotationally symmetrical.
The process is generally less costly than other welding methods and eliminates the need for supplemental operations for machining, cleaning or otherwise modifying the weld junction. Frequently it is possible to eliminate entirely preparation treatments for the surfaces to be welded together and workpieces can be joined in this manner which are sensitive to other welding methods.
In spot-welding or resistance-welding systems, for example, two workpieces are pressed together between electrodes which conduct a current through the workpiece so that the junction is heated by ohmic current losses to form the bond. It is generally necessary, in such cases, to completely clean the surfaces in contact at the interface in a preparation treatment, from contaminants such as grease and even intrinsic metal oxides. The method is not suitable for thick workpieces and frequently causes a change in the metallic structure outwardly of the fusion bond.
Deposition-welding techniques, in which a weldment is supplied to the junction and the latter is heated by torch or electric arc, also have significant disadvantages. In such conventional weld systems, the weld joint may be irregular and thus can require an extensive afterwork, e.g. grinding, to remove excess material or irregularly deposited material.
Resistance welding techniques can eliminate the loss of this often expensive deposited material and the need for significant afterworking. Furthermore, resistance welding can be carried out with high quality machinery since the latter is not endangered by weld spatter or the fumes which may be present upon deposit welding.
Conventional deposit techniques, moreover, operate at temperatures in the region of the melting point of the metals of the workpieces so that there is always the possibility of damage of a structural or internal nature to the workpieces themselves. With friction-welding processes, however, high speeds at the contact interface and the rapidly generated friction heat destroys the oxide layers, breaks down any impurities, and drives the impurities and contaminants outwardly away from the bonding interface. As a result, almost perfect surface contact is brought about between the two workpieces in the bonding region between surfaces that are metallically clean and excluded from contact with the atmosphere. When pressure is then applied transversely to this interface, the surfaces bond together to form practically a defect-free lattice structure.
Friction welding thus requires two purely mechanical processes, namely, friction (brought about by rotation) and compression.
The parameters which are significant are thus friction time, friction force, compression time and compression force, i.e. all mechanical values which can be exactly reproducible, thereby affording the conditions for reproducible repetitive excellent welds with constant reliability.
Friction-welding machines are generally constructed to be able to withstand high loads in the sense of high momentum, speed and mass of the rotating parts and the ability of the machine and both the moving and stationary parts thereof to take up compression forces. In spite of the massive and robust nature of the machines, however, they are able to operate with precisions which cannot be approached by conventional welding processes.
While most friction-welding machines are designed for industrial applications and the mass production of welds in workpieces which are more or less similar, it is frequently desirable for special operations, experimental and test purposes, to provide an increase in the operating range, particularly in terms of the compression force, of the cylinder arrangement which is used.
While it has already been proposed to provide a tandem cylinder arrangement in which a number of cylinders are provided one behind the other, each having a piston acting on the next cylinder, to increase the operating range of a friction-welding machine, such systems have not found widespread use because they take up too much space. It has also been suggested to provide a fluid control system in which the working piston is subjected to counterpressure so that the effective working pressure can thereby be adjusted. Such arrangements are not satisfactory because they are complicated and difficult to handle, and are particularly sensitive.